1. Field of the Invention
The present invention relates to a feedback oscillator. More particularly, it relates to a gallium arsenide field effect transistor feedback oscillator having enhanced efficiency at microwave frequencies.
2. History of the Art
Positive feedback oscillators are well-known in the art and perform a wide variety of essential electrical functions. Such an oscillator comprises, in essence, an amplifier, a frequency determining circuit, and a feedback circuit for feeding a portion of the amplifier output back in phase to the input. Typically the amplifier is a transistor amplifier, and the frequency determining network is an LC, RC or piezoelectric crystal resonant circuit proportioned to resonate at a desired frequency. Such oscillators are used in computers, communications systems, televison systems, industrial control and manufacturing processes, and even in electronic watches.
The design of high frequency power oscillators presents a number of difficult problems. At high frequencies approaching the microwave range, lead wires exhibit substantial inductive reactance, and distributed junction capacitances begin to shunt transistor output. In addition, substantial heat is generated in the transistors in high power oscillators with resulting potential deterioration of device operation and life.
Gallium arsenide field effect transistor oscillators (GaAsFET oscillators) have proven particularly useful as high power, high frequency microwave oscillators. Such oscillators typically utilize a depletion mode Schottky gate GaAs field effect transistor amplifier and a transmission line frequency determining circuit. The transistor is typically connected in a conventionally mounted common gate configuration or a flip-chip mounted common-source configuration.
While such GaAsFET oscillators have provided the highest reported frequencies among microwave devices, they nonetheless exhibit a number of limitations and non-ideal characteristics subject to improvement. For example, while the common-gate configuration has attractive oscillation characteristics, it cannot be readily flip-chip mounted to reduce thermal resistance without including the high parasitics of the heat sinking elements in the frequency determining circuit. And while the common-source configuration can be readily flip-chip mounted, it has less than ideal oscillation characteristics in that (1) the gain decreases rapidly with increasing frequency, and (2) oscillation is critically dependent upon terminal impedances. In addition, the common source configuration typically requires two power supplies of opposite polarity for operation.
Accordingly, there is a need for a GaAsFET oscillator capable of flip-chip mounting and having improved oscillation characteristics.